1. Field of the Invention
The invention relates to a north-referenceable goniometer for azimuthal orientation determination of a sighting device according to the precharacterizing clause of Claim 1, and to a method for azimuthal angle determination relative to the geographical north pole according to the precharacterizing clause of Claim 10.
Accurate referencing of the north direction is necessary in a wide variety of applications. In the area of navigation and orientation, surveying and observation, for example, a north reference which is as exact as possible (within the scope of the desired measurement accuracies) is required. Especially in the field—often under adverse conditions relating to temperature, wind and weather—the north direction should be known as a reference for determining the azimuthal angle, for example for cartographic orientation and location of a target. In what follows, determination of the north direction on the ground, which is not intended for navigation in moving land, air or sea vehicles, will be described. Although the principles described here—particularly relating to the methods used and compensation methods—may in principle also be used for goniometers which are in motion, more highly developed and more complex approaches are required for this, particularly in relation to the mathematical signal evaluation and correspondingly rapid and synchronous signal acquisition.
2. Description of the Background Art
The present invention relates primarily to finding north in goniometers which are supported on a substantially fixed and immobile ground-based surface, so that the described measurements are carried out in fixed relation to the terrestrial coordinate system. These ground-based instruments may however—between use for measurements—nevertheless be mobile in the sense of being portable, i.e. also suitable for changing the site. Requirements resulting therefrom are for example low weight, robustness, battery operation, rapid and simple support and initialization, determination of the north reference in a short time, etc.
For example, the present invention provides north finding in the form of an azimuthally rotatable, north-referenceable subunit for a sighting unit, for instance an observation or measuring instrument, with the aid of which an observation or measurement can be extended or supplemented with reliable and accurate north referencing.
One of the possible applications which may be mentioned for this is a stand or a stand unit, the vertical rotation axis of which (also referred to as the upright axis in surveying) is provided with an angle measurement north-referenceable according to the invention. Various observation instruments can then be mounted on this stand or stand unit, for instance binoculars, monoculars, cameras, distance meters, night vision instruments, moderately sized weapon systems, etc. The north-referenced azimuthal angle measurement according to the invention may, however, as an alternative also be integrated into the sighting device so that only its rotatable base requires ground-based support. Using corresponding data interfaces, information from the sighting device, the azimuthal angle meter according to the invention and other instruments may be combined. For example, the north-referenced azimuthal orientation may be used together with an elevation measurement and a distance measurement in order to determine target coordinates of a sighted object. Furthermore, if the own position is known, for example by means of GPS, the target coordinates may also be determined in a cartographic coordinate system.
Magnetic compass orientations are usually too inaccurate for such purposes, and the known effects of declination and deviation as well as their perturbability by influences of external magnetic and electromagnetic fields usually allow only conditionally accurate north pointings. Furthermore, the accuracy to be expected of a magnetic compass measurement is not predeterminable, and the accuracy of the measurement cannot even be deduced with the aid of the measurement itself. Especially in buildings, steel constructions, tunnels or subterranean devices, and in the proximity of electrical devices, a sufficiently accurate magnetic compass measurement is frequently not possible.
Besides finding magnetic north, it is also known to find north by determining the Earth rotation axis which, by definition, connects the geographical north and south poles. This basic principle has already been known since the discovery of the underlying effect in 1817, particularly since the discovery of the Foucault pendulum by Jean Bernard Leon Foucault in 1851 and the invention of the gyrocompass in 1852, whereupon William Thomson also patented a corresponding compass in 1876. The underlying physical principles are therefore sufficiently known in their basis from textbooks and history books.
The principle has become technically usable more recently above all by further developments in the field of gyroscopic sensors—from the classical gyroscope through the laser-ring and fibre gyroscopes, to the currently known MEMS gyroscopes such as vibration gyroscopes, for example according to the HRG principle (=hemispherical resonant gyroscope) or other known gyroscope technologies. Owing to constant reduction of the overall size and the weight while improving the measurement accuracy and reliability, the use of gyroscopes has also become attractive in portable or mobile instruments. Although the underlying measurement principles are fundamentally very old, constant adaptation has taken place to the components and electronic evaluation and data processing components available in the state of the art, and the characteristics thereof.
For instance, U.S. Pat. No. 4,945,647 discloses a gyrocompass system for ground-based equipment. By means of a high-accuracy inertial sensor, “rapid” and accurate north finding is achieved, which is tolerant to vibrations and settling of the instrument. It contains a ring-laser gyroscope arranged on a rotatable platform, the sensitivity axis of the gyroscope being orthogonal to the rotation axis. A possible oblique setting of the platform is measured in two axes by means of acceleration sensors and correspondingly taken into account in the calculations. The rotatable platform with the gyroscope is contained in a closed housing and is respectively rotated through 90 degrees between the measurements by a motor and indexed there during the measurements.
EP 0 250 608 also describes a method for azimuthal angle determination in which, by means of an acceleration meter, both horizontal orientation of the rotary platform can be carried out and possible sinking during the measurement can be detected and/or numerically compensated for. In order to achieve the required accuracy, the measurement is repeated several times respectively in a position rotated through 180° and is averaged.
EP 2 239 540 describes an instrument comprising a gyroscope for mounting on an accurately levelled goniometer. By means of two gyroscopic measurements in two gyroscope positions rotated through 90 degrees, the instrument can determine the rotation axis of the Earth.
CA 1 269 874 describes a gyrocompass with measurement in three positions respectively offset by 120° with a single gyroscope on a platform which can be moved in a motorized fashion and is levelled on a universal suspension. With the aid of inclination sensors and measurement of the rotational position of the universal suspension with angle meters, the horizontal orientation of the compass is furthermore determined.
Equipment is furthermore known which determines the north direction with a plurality of gyroscopes preferably arranged orthogonally. It is theoretically also possible to determine the north direction with a single non-rotatable gyroscope (more precisely, the east-west direction is determined), but in order to achieve sufficient angular accuracies this also requires high-precision gyroscopes, in particular with very little noise, drift and bias, which are expensive, bulky and heavy and therefore unsuitable especially for economical portable instruments for field use.
It is therefore an object of the present invention to provide an improved north-referenceable azimuthal angle meter, in particular for field use.
It is a further object to achieve sufficient accuracies in this case, i.e. of the order of 1 mil (equivalent to Π/3200 rad), by using economical sensors, especially gyroscopes, of the lower accuracy classes (“tactical grade gyros”).
It is in this case also an object to achieve north referencing with the corresponding accuracy in a short time, preferably in a few minutes or less.
It is a further object to provide an azimuthal angle meter which allows simple and reliable support, and which in particular can be supported without exact horizontal orientation.
It is also an object to use the fewest possible components which are as lightweight and small as possible, in order to achieve high mobility and robust construction for field use. Thus, in contrast to the use of a full-scale 6 DOF navigation unit, the intention is to use only a single gyroscope. Electromagnetic motors are also intended to be obviated in order to obtain a small, lightweight, economical and robust, goniometer.
The provision of an error-tolerant, or error-secure, method for azimuthal angle determination relative to the geographical north pole is also an object, particularly with a known or determinable orientation accuracy being achieved, for example in the form of an accuracy expectation value.
It is also an object to provide an azimuthal angle meter comprising online status monitoring, which informs the user of the need to redetermine the north reference in the event of an (intentional or unintentional) movement or displacement.
These objects are achieved by implementing the characterizing features of the independent claims. Features which refine the invention in an alternative or advantageous way may be found in the dependent patent claims.